1. Field of the Invention
This invention relates to an angle encoder comprising a transmitter disk, which is provided with at least one circular measuring scale, particularly incremental scales, and scanning means, which are operable to scan said at least one measuring scale with reflected light during a rotation of said disk relative to said scanning means and to optoelectronically generate measured-value signals and/or control signals in response to such scanning.
2. Description of the Prior Art
Such angle encoders are used, inter alia, with machine tools, such as lathes, and on automatic manufacturing equipment. In such uses the transmitter disk is preferably mounted directly on a main spindle or motion-transmitting screw of the associated machine and is thus integrated in the machine rather than used as a constituent part of a separate measuring instrument, which has a shaft that serves only to drive the transmitter disk and must be coupled to a mainshaft of the machine in the closest possible alignment therewith. In one possible embodiment one of the circular measuring scale has a very small pitch and comprises, e.g., graduation lines or increments consisting of bright and dark fields in a number of an order of 18,000. In such a case it is possible to take measurements even at high speeds up to, e.g., 12,000 r.p.m. and signals generated in response to the scanning of such scale can be used to monitor the motor speed and the synchronism in machining operations. In addition to the main measuring scale, additional marks may be provided and may also be scanned and may be associated with defined angular positions and may be used, e.g., for a generation of synchronizing pulses. A second circular measuring scale may be provided, which has a larger pitch and comprises preferably 2.sup.n graduations or increments, e.g., 2.sup.10 =1024 graduations or increments, and may be used for a generation of control signals in defined angular positions; such signals are required, e.g., for thread cutting. The small-pitch scale permits an selection of defined angular positions of a spindle, or motion-transmitting screw e.g., when a lathe is used for milling operations.
In arrangements in which transmitter disks are integrated in measuring instruments, which must be coupled to a mainshaft, it is known to use a transmitter disk which is transparent at least at the measuring scales and to provide the measuring scales in two concentric tracks on the end faces of the disk. In that case the radial graduation lines or the empty fields disposed in the incremental scale between such lines will necessarily be wedge-shaped. Such measuring scales can be scanned with transmitted light by means of illuminating means provided on one side of the transmitter disk and scanning units provided on the side and consisting in most cases of photovoltaic cells preceded by gratings for scanning the measuring scale. As the measuring scale may be made in photographic or photolithographic process by means of exposing masks, the costs of making such scales are within tolerable limits. The transmitter disk must be mounted in a separate measuring instrument, which has a shaft that must be coupled to a shaft or spindle or motion-transmitting screw of the machine tool in the closest possible alignment therewith. This is necessary because owing to the wedge shape of the scale elements such angle encoders are highly sensitive to a radial runout. In the selected example of a measuring scale having 18,000 increments the radial runout must not exceed about 1 .mu.m if usable measured-value signals are to be obtained by the scanning of a transmitter disk which is reasonably small in diameter. In case of larger radial runouts, the above-mentioned wedge shape of the scale increments will result in undesired changes in the shape of the signals generated by the scanning (this phenomenon is described as a pumping of the signals) so that such signals may no longer be usable for measuring, positioning and control purposes. To nevertheless permit a mounting of the transmitter disk directly on a shaft or spindle or motion-transmitting screw of a machine tool in such a manner that a larger radial runout will be permissible, it is known to provide angle encoders of the kind described first hereinbefore. In the known angle encoders of that kind the transmitter disk consists of a cylindrical drum, which on its outside cylindrical surface is provided with the two measuring scales, which are scanned with reflected light by the associated scanning units. For that purpose each increment of the measuring scale is composed of a highly reflective field and a less reflective field and the reflected light falls through scanning gratings onto photoelectric receivers. Such known angle encoders have the disadvantage that in the previous practice the measuring scales on each transmitter disk had to be generated directly on said disk so that the manufacturing costs are high, particularly if a small scale pitch is required. Even very small variations of the diameter of the transmitter disks, of an order of micrometers, will result in a change of the perimeter of the cylindrical surfaces which carry the measuring scales and will require a new measuring scale to be generated. Besides, no process has been available thus far which could be used to provide a measuring scale on a cylindrical outside surface by contact printing.